S- to X-Band Stretchable Inductors and Filters for Gigahertz Soft and Epidermal Electronics.

To fulfill the increasing demand for radiofrequency (RF) wireless communication capacity for epidermal electronics, stretchable integrated circuits (ICs) in the gigahertz (GHz) range are desirable. Lumped RF inductors, as a key component in RFICs, typically dominate a large portion of the circuit/chip area and therefore make such inductors mechanically stretchable is critical for GHz-frequency stretchable RFICs. Most of the reported stretchable inductors operate in the MHz frequency range. The only GHz stretchable inductor shows a quality factor of about 2, limiting its potential RF applications. Here, stretchable inductors with a high quality factor of Q > 12.6 and resonance operation frequency of fres > 11.6 GHz are demonstrated by combining microspirals with stretchable structures, overcoming all of the shortcomings of previous demonstrations. Furthermore, a stretchable 1.5-2.6 GHz filter with a peak insertion loss of -2.3 dB at 1.8 GHz is developed, showing negligible performance changes under stretching or on the skin to demonstrate the utility in practical wireless applications like GSM and Bluetooth (2.45 GHz) bands. The demonstrations can facilitate multiple GHz epidermal RFICs in the future.

Heterogeneously integrated flexible microwave amplifiers on a cellulose nanofibril substrate.

Low-cost flexible microwave circuits with compact size and light weight are highly desirable for flexible wireless communication and other miniaturized microwave systems. However, the prevalent studies on flexible microwave electronics have only focused on individual flexible microwave elements such as transistors, inductors, capacitors, and transmission lines. Thinning down supporting substrate of rigid chip-based monolithic microwave integrated circuits has been the only approach toward flexible microwave integrated circuits. Here, we report a flexible microwave integrated circuit strategy integrating membrane AlGaN/GaN high electron mobility transistor with passive impedance matching networks on cellulose nanofibril paper. The strategy enables a heterogeneously integrated and, to our knowledge, the first flexible microwave amplifier that can output 10 mW power beyond 5 GHz and can also be easily disposed of due to the use of cellulose nanofibril paper as the circuit substrate. The demonstration represents a critical step forward in realizing flexible wireless communication devices.